07 July 2009

Some artists create with paint. Some artists create with sound. Some artists create with words. Jerzy Stanislaw Alexandrowicz was an artist who created with methylene blue.

Methylene blue is a straightforward vital stain, and many kids school use it today to see cells under a microscope. But Alexandrowicz took the simple stain to new scientific heights. An obituary gives some idea of his mastery, describing his technique as “legendary,” and his skills so renown that scientists from all over came to visit his lab to learn his methods. (The obituary, incidentally, is well worth reading and paints a picture of a rather remarkable man.)

Alexandrowicz first noted small sensory organs in the tail of crustaceans in the 1930s, but didn’t get to finish the work until after World War II, and published the discovery until 1951. He termed these “muscle receptor organs” (MROs) because the neurons were indeed sitting along a very fine strip of muscle. I doubt the PDF does justice to the pictures in the original paper, but one can get a sense of just how detailed he was. The attentive may object to this being called a “Great moment in crayfish research,” because Alexandrowicz worked with lobsters. Although the muscle receptor organs weren’t originally described in crayfish, almost all of the work that followed was done in crayfish.

From the outset, there was interest in these little sense organs because they were readily accessible, sitting just underneath the exoskeleton and on top of the large fast flexor muscles. In contrast, similar looking organs in mammals, muscle spindle organs, were buried deep in muscles and very difficult to work with. There has been much work on them because of that, and I can only hope to hit a few highlights here.

Cornelius “Kees” Wiersma and colleagues (1953) quickly started doing physiology on these little sense organs, and explicitly drew parallels between them and vertebrate sense organs. They found that the two sensory neurons in the muscle receptor organs had very different properties. One was tonic, responding to slow and small changes, and tended to fire all the time. The other one was phasic, responding only to very hard flexions of the abdomen, and then only for one or two spikes. Wiersma and colleagues continued to do the basic legwork of tracking down the basics of the circuit (Hughes and Wiersma 1960).

Because of the neuronal connections, and the responses of isolated neurons in the dish, Larry Fields (1966) proposed that the muscle receptor organs acted in load compensation (pictured below is Figure 8 from his paper). That is, the muscle receptor organs appeared to be wired so as to detect the difference between how much bending of the tail the animal was trying to do, and how much was actually occurring. If there was a difference between those two, a reflex would kick in, activating extensor muscles to compensate for the impeded movement.

This worked with in a dish.

Unfortunately, when the muscle receptor organs were recorded from live animals performing load compensation, it seemed that the muscle receptor organs didn’t actually work that way (McCarthy and Macmillan 1999). So the function of the muscle receptor organs in intact animals, even after 50 years of work in many labs, still remains to be fully understood.

The muscle receptor organs are an “evergreen” scientific preparation, and I’ve hit on only a very, very small number of papers on them here. That I can only touch on these few highlights in a blog post is a nice example of how often, science is not about breakthroughs, but inch by inch progress.

The red swamp crayfish, Procambarus clarkii, is the most abundant invasive crustacean decapod in Italy. Evidence is however emerging for the presence of other Cambaridae that are erroneously assigned to the P. clarkii taxon. The marbled crayfish, belonging to a still uncertain species of the genus Procambarus, has been found for the first time in Italy in the Canale Maestro della Chiana (Tuscany, Central Italy), where it lives in sympatry with a large P. clarkii population. Although a single specimen was found, this record is particularly relevant due to the parthenogenetic reproductive habit of the marbled crayfish. However, molecular analyses based on COI barcoding did not reveal any differentiation within the P. clarkii population and excluded any form of hybridization between the two species. We will shortly discuss new pathways of invasive species and the threats posed by parthenogenetic species, even though they seem to be still sporadic.